Interference is one of important factors that reduce performance in a wireless network environment. This interference problem mainly occurs in a multi-user environment that transmission-reception pairs exist. This is because that signals simultaneously transmitted from each transmitter are also transferred to non-intended receivers. A technique of interference alignment (IA) has received much attention as a potential method for solving the interference problem. The technique of interference alignment has been developed under a model of an interference channel between X-channel and K-user, and has been developed for a practically-relevant network model related to various actual statues. Particularly, under a cellular network status, a technique of interference alignment, which achieves almost the same performance as degree of freedom (DoF) performance in an interference-free status, has been developed.
Although these techniques of interference alignment have been contributed to increase of communication capacity, various problems exist to apply the techniques to the actual environment. One of the problems is that a transmitter should exactly know current channel state information (CSI). In a frequency division duplex (FDD) communication system of the related art, this channel information is generally fed back from a receiver to the transmitter. In this case, delay additionally occurs to receive the feedback. Therefore, in a communication system of the related art, current channel information is predicted based on outdated channel state information (outdated CSI) and then a technique of interference alignment based on the predicted channel information has been applied. However, in a fast-fading scenario in which a channel environment is fast changed, a current channel state may completely be different from a channel state predicted from the feedback. In this case, a method for predicting a current channel state is failed to improve communication capacity (improve DoF gain).
However, according to the result published in the paper suggested by Maddah-Ali and Tse, communication capacity could be improved even with outed channel state information in a multi-antenna broadcast channel Particularly, these people have developed an innovative transmission method. This technique may considerably increase communication capacity by combining the past received signals in each receiver to generate assistance signals and transmitting the generated signals to several receivers at the same time.